Inside Macintosh: Imaging with QuickDraw
The DeviceLoop procedure searches for graphics devices that intersect your window's drawing region, and it informs your application of each different graphics device it finds. The DeviceLoop procedure provides your application with information about the current device's pixel depth and other attributes. Your application can then choose what drawing technique to use for the current device. For example, your application might use inversion to achieve a highlighting effect on a 1-bit graphics device, and, by using the HiliteColor procedure described in the chapter "Color QuickDraw," it might specify a color like magenta as the highlight color on a color graphics device.
For example, you can call DeviceLoop after calling the Event Manager procedure BeginUpdate whenever your application needs to draw into a window, as shown in Listing 1 .
Listing 1 Using the DeviceLoop procedure
PROCEDURE DoUpdate (window: WindowPtr);
VAR
windowType := Integer;
myWindow: LongInt;
BEGIN
windowType := MyGetWindowType(window);
CASE windowType OF
kSimpleRectanglesWindow: {simple case: window with 2 color rectangles}
BEGIN
BeginUpdate(window);
myWindow := LongInt(window); {coerce window ptr for MyDrawingProc}
DeviceLoop(window^.visRgn, @MyTrivialDrawingProc,
myWindow, []);
EndUpdate;
END;
{handle other window types--documents, dialog boxes, etc.--here}
END;
When you use the DeviceLoop procedure, you must supply a handle to a drawing region and a pointer to your own application-defined drawing procedure. In Listing 1 , a handle to the window's visible region and a pointer to an application-defined drawing procedure called MyTrivialDrawingProc are passed to DeviceLoop . For each graphics device it finds as the application updates its window, DeviceLoop calls MyTrivialDrawingProc .
Because DeviceLoop provides your drawing procedure with the pixel depth of the current device (along with other attributes passed to your drawing procedure in the deviceFlags parameter), your drawing procedure can optimize its drawing for whatever type of video device is the current device, as illustrated in Listing 2 .
Listing 2 Drawing into different screens
PROCEDURE MyTrivialDrawingProc (depth: Integer;
deviceFlags: Integer;
targetDevice: GDHandle;
userData: LongInt);
VAR
window: WindowPtr;
BEGIN
window:= WindowPtr(userData);
EraseRect(window^.portRect);
CASE depth OF
1: {black-and-white screen}
MyDraw1BitRects(window); {draw with ltGray, dkGray pats}
2:
MyDraw2BitRects(window); {draw with 2 of 4 available colors}
{handle other screen depths here}
END;